The present invention relates generally to a method for manufacturing articles or elements of pressure agglutinated, agglomerated, coalesced or consolidated powder, hereinafter referred to as "compacted powder" or "compact", obtained by compressing a predetermined amount of powder in molds.
The said method relates more particularly to the manufacture of unit charges of compacted powder for small-, medium- and large-caliber arms ammunition and small self-propelling appliances. The invention, however, also applies to the manufacture of any unit articles or elements of compacted powder, e.g. of metal, metal alloy or any other material in powder form, which are obtained by filling molds with powder in bulk and then compressing the same to form unit elements of compacted powder.
The molds used in the prior art are metal molds constituted by hollow cylinders at least one end of which is closed by a movable punch of a press. Each mold is filled with powder and then the punch or punches actuated by the press move within the mold and compress the powder. Very good dimensional characteristics are thus obtained, as the mold and the punch or punches are substantially indeformable. However, as a direct result of the mechanical compression of the powder within the molds by means of the punch or punches, the density of the compact resulting from the compression is not uniform throughout the mass of the article thus obtained. Where the articles are unit charges of compacted propelling powder, this final poor homogeneity results in an uneven combustion of the powder and therefore a deterioration of the firing or shooting characteristics.
Moreover, the mechanical compression of the powder in a metal mold in a press requires considerable investment and the production rates are very low. The manufacture of such articles therefore proves to be highly expensive.
Attempts have therefore been made, in the manufacture of unit articles or elements of compacted powder, to replace mechanical compression in metal molds and by means of metal punches by isostatic pressing in molds of flexible and/or resilient material, which are filled with powder, closed substantially sealingly and placed in a pressure chamber filled with a substantially incompressible fluid in order to be subjected to isostatic compression. It has been found that much better results can thus be obtained from the point of view of the final homogeneity of the elements (and more particularly a highly even density, therefore a highly even combustion and very good firing characteristics) and of the production rates, which are extremely improved compared with those of the mechanical presses, while at the same time considerably reducing investments.
However, such important improvements in the final density of the articles and in production rates are obtained to the detriment of the dimensional characteristics. These, which were excellent in the mechanical compression method, are much poorer in the case of isostatic compression for the molds of flexible and/or resilient material are necessarily deformed during the compression. It has been found that this deformation is uneven and much more marked in the intermediate region of the molds than in its sealingly closed end regions, so that the compacted powder articles obtained do not have a perfectly straight cylindrical shape but are smaller in diameter in their intermediate region than at their ends.